Radio Resource Control (RRC) is a signaling protocol which uses lower layers for segmentation and reliable in-order delivery of signaling messages. RRC is suitable for messages of any size requiring reliable delivery such as user equipment (UE) configuration. In Long Term Evolution (LTE) and LTE-advanced (LTE-a), RRC is involved in the Non-Access Stratum (NAS) message exchange between a UE and a Mobility Management Entity (MME) as well as provide various control-plane functions both on the UE and the evolved NodeB (eNodeB or, in short, eNB).
These control-plane functions include, on the eNB side, connection management, radio resource management, measurement configuration and mobility control, service management and security, system information broadcast and idle mode mobility. On the UE side, these control-plane functions include idle mode mobility, for example. To be more specific, in connection management RRC may play a role in RRC connection establishment, maintenance and release, radio bearer connection establishment, maintenance and release, and multi-cell and multi-node configurations. In radio resource management, RRC may be involved in configuration of radio resources for RRC connection and configuration of lower layers and in radio configuration control including e.g., assignment and/or modification of Automatic Repeat Request (ARQ) configuration, Hybrid Automatic Repeat Request (HARQ) configuration, and Discontinuous Reception (DRX) configuration. Still further, in measurement configuration and mobility control, RRC may be used in UE measurement reporting and control of the reporting and mobility functions (intra- and/or inter-frequency handover, and inter-Radio Access Technology (inter-RAT) handover). In service management and security, RRC may be involved in Multimedia Broadcast Multicast Service (MBMS) services, Quality of Service (QoS) management functions and Access Stratum (AS) security, and, in system information broadcast, RRC may be involved in Non-Access Stratum (NAS) (on idle mode mobility management and service setup) as well as AS (on data transfer and RRM). In idle mode mobility, RRC may be used in paging, cell reselection and system selection.
Typically, when a new Radio Access Technology (RAT) is standardized, this is done by also introducing a separate Core Network catering for that RAT and 3GPP introduces mechanisms to move from one RAT to another RAT with minimal service interruption via the Core Network. Hence, in any case, moving from one RAT to another RAT means establishing a RRC connection towards the target RAT and removing the RRC connection from the source RAT, and because those RRC connections terminate in different logical nodes anchoring in different Core Networks (i.e., are completely separate UE connections), there is no possibility of synergy between them. This also implies a very high amount of signalling between the source and the target RAT, and higher risks of call drops.
Thus, when a UE moves from one RAT to another RAT, the UE needs to release the RRC connection in the source RAT and establish a new RRC connection in the second RAT. The RRC connection procedure is identical to the procedure happening when a UE initially connects to the second RAT. This procedure is quite intense from the control signaling load point of view, and it also adds a delay.